Why do you want to use prototypes? They almost never do just what you want and/or expect. In any event, optional parameter values must be handled in the same way: by checking in the function that a defined (or otherwise valid) value has been passed.

>perl -wMstrict -le
"sub opt_w_proto ($;$) {
   my ($x, $y) = @_;
   $y //= 42;
   print qq{x $x  y $y};
   }
 ;;
 sub opt {
   my ($x, $y) = @_;
   $y //= 42;
   print qq{x $x  y $y};
   }
 ;;
 opt_w_proto(7);
 opt_w_proto(7,0);
 ;;
 opt(7);
 opt(7,0);
"
x 7  y 42
x 7  y 0
x 7  y 42
x 7  y 0
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